It is well known that internal combustion engines can produce harmful chemical species in their exhaust streams. It is therefore desirable to eliminate or at least reduce such pollutants to levels low enough that human health is not adversely affected. As a result of the high temperatures that are reached during a combustion event, many chemical species are produced from the oxidation of hydrocarbon fuels, including the oxides of nitrogen (NO and NO2, collectively referred to as NOx). Due to their impact on human health, many countries in the global community have enacted legislation that seeks to limit the emission of NOx from both mobile and stationary sources, and many techniques have been developed to achieve this objective. Among these, the use of catalysis technology has been found to be particularly effective and economically viable, however, it should be noted that different approaches are needed when treating the oxygen-rich exhaust streams from so-called “lean” combustion than is the case for stoichiometric combustion exhaust streams. Examples of lean combustion NOx sources include the compression ignition or diesel engine and the direct-injected lean-burn spark ignition or gasoline engine.
Lean-burn engines are unable to take advantage of the well developed and effective 3-way catalyst systems that are universally used by homogeneous spark ignition engines. Accordingly, the remediation of NOx for lean-burn engines requires the addition of a reductant in conjunction with a suitable catalyst. The reduction of NOx requires near real-time dosing control since NOx production closely follows engine load but is moderated by the amount of ammonia already stored on the catalyst. Accordingly, the reductant dosing schedule is a highly dynamic activity.
Under steady state operating conditions, with a warmed-up engine/catalyst system, it is relatively easy to match the reductant dosing rate to the engine NOx production rate and thereby achieve very high conversion ratios of about 98%. However, under transient operating conditions, achieving this match is much more challenging due to catalyst temperature variation and NH3 storage effects such that conversion ratios of 85 to 90% are more typically achieved. Therefore, quantitative accuracy in dosing and responsiveness to load changes are key requirements for such a system.
One very effective technology for the remediation of NOx in an oxygen-rich exhaust stream is the technique widely known as Selective Catalytic Reduction (hereafter referred to as SCR). In this approach, an ammonia-containing reagent (or reductant) is injected into an exhaust stream at a rate closely related to the instantaneous NOx content of that stream wherein the ammonia (NH3) reacts with the NOx in conjunction with a vanadia-based or similar catalyst such that the pollutant is converted to harmless nitrogen (N2) and water in the tail gas. Both selective catalytic reduction and selective non-catalytic reduction (SNCR) have been used extensively in the industrial sector, and SCR systems have recently been subject to development for mobile emission sources.
Notwithstanding the above, existing SCR dosing systems have a variety of shortcomings.
In prior art systems, a pressurizing pump is located in or near the tank module in order to supply reductant at a fixed known pressure to a remote injector nozzle adjacent the SCR catalyst. This system operates according to a so-called pressure/time metering principle whereby reagent metering is achieved by exposing the control orifice to the controlled pressure for a known time duration. In order to achieve the necessary stability of pressure control in such a system requires sophistication, and therefore expense. Moreover, metering accuracy is dependant on stability of the atomizing nozzle flow area which, due to its location in the hot exhaust environment, is susceptible to change due to crystallized urea deposit build-up.
Many known systems utilise return flow architectures whereby surplus reagent above and beyond that which is needed for SCR dosing is supplied to the nozzle purely for cooling purposes, whence it is returned through a separate duct back to the storage tank. Additionally, such systems may require a purge feature which is activated on engine shut-down which minimizes the propensity for nozzle clogging from salt precipitates due to heat soak-back into the nozzle assembly from the hot exhaust. In such cases, a separate purge pump is required and, since the purge is carried out after the engine is switched off, the purge pump causes an undesirable drain on the vehicle battery.
Examples of existing dosing systems having these features are described in WO 2006/050547 (Pankl Emission Control Systems) and WO 2008/006840 (Inergy Automotive Systems Research).
Positive displacement pumps for reagent dosing are known in the prior art, for example in WO 2005/024232 (Hydraulic Ring), WO 2007/071263 (Grundfos) and WO 2008/031421 (Thomas-Magnete). However, such pumps have been designed to operate at undesirably low pressures and, because the pumping plunger is separated from the reagent by a flexible diaphragm, the accuracy of reagent volumetric quantity metering is also of a much lower order than can be achieved by a piston pump.
It is an object of the present invention to provide an SCR dosing system which substantially overcomes or mitigates the aforementioned problems. It is a further object of the invention to provide an advantageous method of operating an SCR dosing system.